Polyhydric alcohols or polyols possess considerable economic significance as a condensation component for forming polyesters or polyurethanes, synthetic resin coatings, lubricants and plasticizers. In this context, polyhydric alcohols of interest are particularly those which are obtained by a mixed aldol addition of formaldehyde with iso- or n-butyraldehyde. The aldol addition between formaldehyde and the appropriate butyraldehyde first forms an aldehydic intermediate which then has to be reduced to the polyhydric alcohol. An industrially important polyhydric alcohol obtainable by this method is neopentyl glycol [NPG, 2,2-dimethylpropane-1,3-diol] formed from the mixed aldolisation of formaldehyde and isobutyraldehyde.
The aldol addition reaction is carried out in the presence of basic catalysts, for example alkali metal hydroxides or aliphatic amines, and initially affords the isolable hydroxypivaldehyde (HPA) intermediate. This intermediate can subsequently be converted with excess formaldehyde in accordance with the Cannizzaro reaction to neopentyl glycol to form one equivalent of a formate salt. In this configuration of the reduction step, the formate salt is therefore obtained as co-product and the cost-effectiveness of this method variant also depends on the commercial opportunities for the formate salt.
However, also implemented industrially is the catalytic hydrogenation of hydroxypivaldehyde in the gas and liquid phase over a metal catalyst. In the gas phase variant, hydroxypivaldehyde is initially freed from high boilers in an evaporator connected upstream of the hydrogenation stage. The subsequent hydrogenation is preferably conducted in the presence of Raney nickel catalysts or supported catalysts based on nickel which may additionally comprise further active metals such as copper or chromium and, additionally, activators. The gas phase variant is covered, for example, in EP 0 278 106 A1; U.S. Pat. No. 4,094,914; Ullmann's Encyclopedia of Industrial Chemistry, publisher VCH, 5th Ed., 1985, Vol. A1, p. 308; Chemiker-Zeitung (Chemist journal), volume 100, (1976), No. 12, pp. 504-514.
The hydrogenation in the liquid phase has been extensively described, for example in EP 0 484 800 A2, using catalysts based on copper, zinc and zirconium. The liquid phase hydrogenation of hydroxypivaldehyde is frequently conducted in the presence of copper chromite catalysts. Copper chromite catalysts frequently comprise other metals as activators, for example barium, cadmium, magnesium, manganese and/or a rare earth metal. According to U.S. Pat. No. 4,855,515, manganese-doped copper chromite catalysts in particular excel at the hydrogenation of the aldolisation product of the reaction of formaldehyde with isobutyraldehyde. WO98/29374 A1 discloses the use of a barium-doped copper chromite catalyst for the hydrogenation of hydroxypivaldehyde in a methanolic solution.
According to the teaching of DE 1 518 784 A1, a mixture of hydroxypivaldehyde and excess isobutyraldehyde is hydrogenated to neopentyl glycol and isobutanol in the presence of a copper chromite catalyst which has been doped with barium. According to EP 0 006 460 A1, the two-step high pressure hydrogenation of crude hydroxypivaldehyde, which is carried out with increasing hydrogenation temperatures, also uses a copper chromite catalyst activated with barium.
EP 0 522 368 A1 discloses carrying out the hydrogenation of hydroxypivaldehyde in a solution comprising at least 20% by weight of a low molecular weight alcohol, for example methanol or n-butanol, based on the mixture of alcohol and reaction product, and also water in an amount of not more than 40% by weight, based on the total amount of water, alcohol and reaction product. The hydrogenation catalyst recommended is a copper chromite catalyst.
As a product produced industrially, neopentyl glycol has a major economic significance and thus there always exists a need to improve the known methods for preparing neopentyl glycol, whether by improving the product yield, by better utilization of plant equipment or by a lowering of energy input.